Method for producing a compound from a flowable plastic material and a solid fiber core by means of extrusion and device for carrying out said method

ABSTRACT

Continuous fibers are led through a roller nip which is formed between a pair of wetting rollers which are driven in opposite rotational directions and the surface of which has a coating of the flowable plastic material in a thickness which is sufficient for the constant filling of the roller nip with the plastic material. The wetted fibers are fed to the plastic material in the extrusion chamber, and the continuous fibers are mixed with the plastic material in the extrusion chamber. A device for carrying out the method includes a screw-type extruder having an extrusion chamber surrounded by an extruder barrel which has, in addition to the entry opening and the exit opening for the extruded plastic material, a lateral opening for the feeding in of continuous fiber material, and a wetting device for the wetted fiber material to be fed in, the outlet of which is in transporting connection with the lateral opening.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for producing a compound from aflowable plastic material and a solid fiber core wherein continuousfibers of reinforcing material which are wetted with plastic materialare fed to the plastic material in the mixing zone of an extrusionchamber. Furthermore, the invention relates to a device for carrying outthis method.

2. Description of the Related Art

U.S. Pat. No. 6,221,293 discloses a method for producing a compound froma plastic material and a solid fiber core in which a heated screw-typeextruder is used, into which continuous fibers are drawn by drawingthemselves in, are cut up there and mixed with the plastic material, sothat subsequently a finished fiber compound can be delivered. To be ableto keep the length of the mixing zone required for intensive mixing withthe flowable plastic material in the extruder as short as possible, thecontinuous fibers are wetted in an impregnating die with the plasticmaterial being used before they enter the extruder.

With regard to the impregnating die, reference is made to a commerciallyavailable device which is described in U.S. Pat. No. 4,883,625. Thisknown device for wetting continuous fiber material is designed in theform of an extrusion die which has a continuous channel for the leadingthrough of the fiber material. In the initial region of this channel,two flow channels for the feeding of flowable plastic material to thecontinuous fiber material end. The channel does not proceed in astraight line in the longitudinal direction, but instead has a number ofkinks in the downward or upward direction, so that, when it is being ledthrough, the fiber material is in each case deflected aroundcorresponding edges. The intended effect of this is to ensure that thefiber material is wetted as uniformly and completely as possible.deflected around corresponding edges. The intended effect of this is toensure that the fiber material is wetted as uniformly and completely aspossible.

SUMMARY OF THE INVENTION

The object of the present invention is to develop a method of thegeneric type to the extent that uniform and complete wetting of thefiber material is ensured in as simple a way as possible. At the sametime, it is intended that a compound which deliberately contains aparticularly high proportion of very long fibers can be produced, toensure a high breaking strength of the finished parts which can beproduced from this compound. Furthermore, a device for carrying out thismethod is to be specified.

The invention is based on the idea that the production of a compoundfrom a flowable thermoplastic material and a fixed fiber core of areinforcing material takes place by extruding the materials being usedfrom an extrusion chamber. The flowable plastic material may inprinciple also be thermosetting liquid polymers such as for instancepolyester resins. However, thermoplastic which are plasticated in theextrusion chamber by supplying heat, for example polyethylene orpolypropylene, are preferably used. Any desired materials may be used asthe fiber material for reinforcing the flowable plastic material used inthe sense of a matrix material. They are preferably glass fibers. Thefiber reinforcing material is fed in the form of continuous fibers tothe plastic material which is located in the extrusion chamber andalready flowable and is mixed with the latter. Before the continuousfibers are introduced into the extrusion chamber, they are wetted withan amount of the intended flowable plastic material that is small incomparison with the amount of plastic material in the extruder. Thisprior wetting makes it possible to minimize the mixing work in theextruder to achieve a distribution of the fibers that it is as uniformas possible. As a result, the fibers fed in, which tend to break or tearunder the action of the mixing tool used, can maintain a comparativelygreat length. The longer and more intensely the mixing work is carriedout, the shorter the individual pieces of fiber become. In order thatthe products produced by further processing from the compound producedaccording to the invention achieve as high a breaking strength aspossible, it is important however that the length of the fibers remainsas long as possible. Without adequate wetting of the continuous fibersfed in, a much longer mixing operation would have to be carried out.

To achieve complete wetting of the continuous fibers with the flowableplastic material, the invention provides that the continuous fibers areled through a roller nip which is formed between a pair of wettingrollers driven in opposite rotational directions. In this arrangement,the surface of these wetting rollers has a coating of the plasticmaterial of the matrix material in flowable form in a thicknesssufficient for the constant filling of the roller nip with the plasticmaterial. By leading the continuous fibers through the roller nip, thecontinuous fibers are inevitably brought into extremely intensivecontact with the flowable plastic material and are consequently wettedcompletely with the plastic material on the surface in a veryuncomplicated way.

In principle, it is possible to produce the flowable plastic materialfor coating the wetting rollers with a separate extruder and feed it tothe surface of the wetting rollers. However, it is particularlyadvantageous to take the flowable plastic material required for thispurpose from the extrusion chamber used for mixing with the continuousfibers as a partial stream at a point at which the plastic material isalready flowable but has not yet been mixed with the fibers.

On account of their particularly good mixing properties, multi-screwextruders, preferably twin-screw extruders, are suitable for theproduction of the compound. In principle, however, single-screwextruders can also be used. The feeding in of the wetted continuousfibers expediently takes place in the region of the mixing zone of suchan extruder. This mixing zone need have only a comparatively shortlength for adequate mixing with the matrix material.

To achieve particularly good wetting of the continuous fibers, it isrecommendable to operate the wetting rollers in each case withcircumferential roller speeds that are different from each other. Inthis way, a friction occurs in the roller nip, leading to fibrillationof the fiber material. This allows the individual fibers of the fiberstrands fed in to be brought into particularly intensive contact withthe flowable plastic material in the roller nip and be completelyenveloped. The fibrillation successfully allows fibers that are stickingtogether, for instance due to “coating”, to be broken up and to achievewetting of the “individual fibers”. Different circumferential rollerspeeds can be achieved for example with the same rotational drive speedby choosing correspondingly different roller diameters. Thecircumferential roller speeds are realized, however, by correspondingsetting and regulation of different rotational speeds of the wettingrollers.

To ensure orderly feeding of the continuous fibers to the wettingrollers, it is recommendable to lead the continuous fibers initiallyover a pair of drawing-in rollers, rolling on each other, and to passthem from these rollers into the roller nip of the wetting rollers. Thedrawing-in rollers, like the wetting rollers, each rotate oppositely inrelation to the other, in order to achieve the material transport in thedesired direction.

The device according to the invention for carrying out the methoddescribed above has as a major part a screw-type extruder, the extrusionchamber of which, surrounded by an extruder barrel, has in addition tothe two openings for the entry of the plastic material of the matrixmaterial and the exit opening for the extruded material a furtherlateral opening which serves for the feeding in of the continuous fibermaterial. The opening for the fiber material consequently lies betweenthe entry opening and the exit opening for the plastic material, to beprecise generally much closer to the exit opening. A further importantpart of the device is the wetting device for the fiber material. Theoutlet of this wetting device is in this case in transporting connectionwith the opening for the continuous fiber material in the extruderbarrel. According to the invention, the wetting device has at least onepair of interacting wetting rollers, which are driven such that theyrotate in opposite directions. In principle, it is also possible toarrange a number of pairs of such wetting rollers one behind the otheror one next to the other. However, in general it is advisable to limitthe wetting rollers to a single pair. The roller nip existing betweenthe wetting rollers forms the outlet of the wetting device. In theregion of the periphery of the wetting rollers, i.e. in the vicinity oftheir circumference, a feed for the flowable plastic material requiredfor the wetting is arranged in such a way that the outer surface of thewetting rollers is constantly coated with the flowable plastic materialduring operation.

It is recommendable in the interests of a compact design to mount thewetting rollers in an enclosing housing which is firmly connected to theextruder barrel. This consequently means that this housing is arrangedin the region of the lateral opening for the feeding in of the fibermaterial. The lateral opening is expediently provided in the region ofthe mixing zone of the screw-type extruder. In front of the housing inwhich the wetting rollers are mounted there is preferably also arrangeda pair of drawing-in rollers, driven such that they rotate in oppositedirections, by means of which the continuous fibers of the fibermaterial can be led in a controlled manner into the roller nip of thewetting rollers.

In principle, the screw-type extruder may have a single screw in theextrusion chamber. Better mixing is achieved, however, on multi-screwextruders. The use of twin-screw extruders is particularly preferred,the screws of the twin-screw extruder preferably being driven such thatthey rotate in opposite directions.

To create friction in the roller nip to assist complete wetting of thefiber material, in principle wetting rollers with different diametersmay be used, so that they can be driven at the same rotational speedalthough different circumferential speeds at the outer roller surfaceare desired. It goes without saying that it is also possible withdiameters of the wetting rollers that are the same as one another fordesired different circumferential roller speeds to be achieved by adifferent gear transmission in the drive of the individual rollers. Itis recommendable to design the rotary drives of the wetting rollers insuch a way that their circumferential speed can be set and regulatedindependently of one another. This can be achieved in the simplest wayby the rotary drives of the wetting rollers each being designed suchthat they are completely separate from one another.

With regard to the feed for the flowable plastic material to the wettingrollers, it is regarded as advantageous to design these feeds each as abranch channel which branches off upstream in the transporting directionof the screws of the screw-type extruder of the opening for the feedingof the fiber material from the extrusion chamber and ends openly in thedirect proximity of the outer surface of the respectively assignedwetting roller, so that, as a consequence of the pressure in theflowable plastic material brought about by the screw or screws, apartial stream of this plastic material is applied through the branchchannel to the outer surface of the wetting rollers.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a schematic cross-section of an extruder according tothe invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The FIGURE shows in cross section a screw-type extruder 1, which isdesigned as a twin-screw extruder with the two screws 11 and 12. Asindicated by the two rotational arrows entered, the two screws 11, 12are driven in a preferred way such that they rotate in the samedirection. The extrusion chamber of the screw-type extruder is denotedby the reference numeral 2, while the extruder barrel bears thereference numeral 8. In the region of the mixing zone of the screw-typeextruder 1, the extruder barrel 8 has a lateral opening 3 toward theextrusion chamber 2. Arranged above this opening 3 is a housing 7, whichis firmly screwed directly to the extruder barrel 8. Rotatably mountedin this housing 7 are two wetting rollers 5, 6, which have a cylindricalouter surface and the same diameter. The axes of rotation of thesewetting rollers 5, 6, which belong to the wetting device 4, lie parallelto the longitudinal axis of the two screws 11, 12. They are driven byseparate, variable-speed motor drives in opposite directions ofrotation, the rotational speed of the wetting roller 5 being set to beslightly different than the rotational speed of the wetting roller 6, tocreate a certain degree of friction in the roller nip of the wettingrollers 5, 6 lying close to each other. The rotational direction of thewetting rollers 5, 6 and the roller nip are set such that a strand ofcontinuous fibers of a glass fiber material, which is not represented inany more detail but is fed in the direction of the depicted arrow overthe wetting device 4, is conveyed in a specifically directed mannerthrough the opening 3 into the extrusion chamber 2. To accomplishcontrolled feeding of the glass fiber material, two drawing-in rollers9, 10 are also rotatably arranged above the two wetting rollers 5, 6 andare driven such that the strand of the continuous fibers enters theroller nip of the wetting rollers 5, 6 in a straight line. In order thatthe desired complete wetting with the flowable plastic material of thematrix material takes place in the roller nip between the two wettingrollers 5, 6, the outer surface of the wetting rollers 5, 6 must in eachcase be coated with an adequate amount of flowable plastic material forthe latter to fill the roller nip sufficiently. Provided for thispurpose in an advantageous development of the invention there is in eachcase a branch channel 13, 14, which branches off from the extrusionchamber 2 in a way not shown at a point where already flowable plasticmaterial is present in the extrusion chamber 2, and leads through theextruder barrel 8 and through the frame 7 to the outer surface of therespective wetting roller 5, 6 and openly ends there. As a result, apartial stream of the flowable plastic material can be branched off fromthe extrusion chamber 2 and led up to the wetting rollers 5, 6. Theflowable plastic material is discharged at the outer surface of thewetting rollers 5, 6, i.e. it issues into the nip between the outersurface of the wetting rollers 5, 6 and the corresponding cylindricalrecess in the frame 7, in which the two wetting rollers 5, 6respectively run. As a result, the flowable plastic material distributesitself in a uniform thickness under constant rotation of the two wettingrollers 5, 6 on their outer surface. The material feed through the twobranch channels 13, 14 is adjusted such that the roller nip between thetwo wetting rollers 5, 6 is adequately filled when the strand ofcontinuous fibers is led through.

As a result of the friction and compressing processes in the roller nip,very intensive contact occurs between the fiber material and the plasticmaterial of the matrix material, so that complete wetting of the surfaceof the fiber material is ensured. The excess material of the flowableplastic material is entrained by the wetting rollers 5, 6 and can bestripped off without any problem at a corresponding edge inside theframe 7, so that it falls into the opening 3 on the extruder barrel 8and is inevitably taken along by the screws 11, 12 of the extruder withthe fiber material introduced into the opening 3 and mixes again withthe main stream of plastic material. The fiber material can be conveyedinto the extrusion chamber 2 in a very uniform way by the drawing-inrollers 9, 10 and the wetting rollers 5, 6.

The mixing action of the two extruder screws 11, 12 does admittedlycause repeated breakages in the fiber material. Since, however, themixing zone for the uniform distribution of the fiber material in thematrix material of the flowable plastic material can be relativelyshort, the continuously fed-in fibers are predominantly maintained in acomparatively great length. The proportion of short fibers isconsiderably less than in the case of the previously known methods.Since the device according to the invention merely comprises individualparts which can be produced very easily and the required rotary drivesare obtainable at favorable prices, the overall expenditure for settingup a device according to the invention is very low.

What is claimed is:
 1. A method for producing a compound from a flowableplastic material and a solid fiber core of reinforcing material, saidmethod comprising: extruding the plastic material from an extrusionchamber, providing a pair of wetting rollers which are driven inopposite rotational directions, said wetting rollers having respectivesurfaces which form a nip between said rollers, coating said surfaceswith a flowable plastic material in a thickness which is sufficient toconstantly fill the roller nip with plastic material, feeding continuousfibers of reinforcing material through said nip so that said fibers arewetted with plastic material, feeding the wetted fibers to the plasticmaterial in the extrusion chamber via a lateral opening in the extrusionchamber, and mixing the wetted fibers with the plastic material in theextrusion chamber.
 2. A method as in claim 1 wherein flowable plasticmaterial is taken from said extrusion chamber at a point upstream ofsaid lateral opening and used to coat said surfaces of said rollers. 3.A method as in claim 1 wherein said mixing occurs in the mixing zone ofa multi-screw extruder.
 4. A method as in claim 1 wherein said flowableplastic material is a plasticated thermoplastic.
 5. A method as in claim1 wherein said wetting rollers are operated with different surfacespeeds to create friction in the nip.
 6. A method as in claim 5 whereinsaid wetting rollers are driven at different rotational speeds.
 7. Amethod as in claim 1 wherein said continuous fibers are fed to said nipin a controlled manner by means of a pair of drawing-in rollers rollingon each other.
 8. A device for producing a compound from a flowableplastic material and a solid fiber core of reinforcing material, saiddevice comprising: a screw-type extruder comprising an extrusion chambersurrounded by an extruder barrel, said extrusion chamber having an entryopening for feeding in flowable plastic material, an exit opening forextruded plastic material, and a lateral opening for the feeding in ofcontinuous fiber material, a wetting device comprising a pair of wettingrollers which are driven in opposite rotational directions, said wettingrollers having respective surfaces which form a nip between saidrollers, said nip forming an outlet which is in transporting connectionwith the lateral opening of the extrusion chamber, and means forconstantly coating the surfaces of the wetting rollers with a flowableplastic material.
 9. A device as in claim 8 further comprising anenclosing housing in which said wetting rollers are mounted, saidenclosing housing being fixed to said extruder barrel.
 10. A device asin claim 8 further comprising a pair of drawing-in rollers which aredriven to rotate in opposite directions, said drawing-in rollers beingarranged for feeding continuous fibers of fiber material into the nip ofthe wetting rollers in a controlled manner.
 11. A device as in claim 8wherein said screw-type extruder is a twin-screw extruder.
 12. A deviceas in claim 11 wherein said twin-screw extruder comprises a pair ofscrews which are driven to rotate in the same direction.
 13. A device asin claim 8 further comprising means for driving said wetting rollers sothat the surfaces have different speeds thereby to create friction inthe nip.
 14. A device as in claim 13 wherein said wetting rollers havethe same diameter, said means for driving said wetting rollerscomprising means for regulating their rotational speeds independently ofeach other.
 15. A device as in claim 14 wherein said means for drivingsaid wetting rollers comprises a respective rotary drive for each saidwetting roller.
 16. A device as in claim 8 further comprising a pair ofbranch channels which branch off of said extrusion chamber upstream ofsaid lateral opening and end in the direct proximity of the surfaces ofthe respective wetting rollers.
 17. A device as in claim 8 wherein saidlateral opening is arranged at the mixing zone of the screw-typeextruder.